1. Field of the Invention
The present invention relates to a thin film transistor substrate that can be applied to a display device such as a liquid crystal display device, and more particularly, to a thin film transistor that crystallizes an active layer by using a laser.
2. Discussion of the Related Art
A thin film transistor has been widely used as a switching device of a display device such as a liquid crystal display device.
The thin film transistor includes a gate electrode, an active layer, and source and drain electrodes, and may be divided into a staggered structure and a coplanar structure depending on arrangement of the electrodes.
In the staggered structure, the gate electrode and the source/drain electrodes are arranged up and down in regard to the active layer, and in the coplanar structure, the gate electrode and the source/drain electrodes are arranged on the same plane.
The thin film transistor of the staggered structure may be divided into a back channel etched (BCE) type thin film transistor and an etch stopper (ES) type thin film transistor depending on a method for forming a channel.
In the back channel etched type thin film transistor, a channel region of a semiconductor layer is etched during an etching process for forming the source and drain electrodes, and in this case, a problem occurs in that the active layer may be too etched. On the other hand, in the etch stopper type thin film transistor, since an etch stopper is formed on a semiconductor layer, a channel region of the semiconductor layer is not etched during an etching process for forming the source and drain electrodes, whereby there is no concern that the active layer may be too etched.
Meanwhile, in manufacturing the etch stopper type thin film transistor, there is provided a method for crystallizing the active layer by using a laser. Hereinafter, the etch stopper type thin film transistor (hereinafter, referred to as ‘thin film transistor’) according to the related art will be described with reference to the accompanying drawings.
FIGS. 1A to 1E are cross-sectional views illustrating process steps of manufacturing a thin film transistor substrate by crystallizing an active layer using a laser according to the related art.
First of all, as shown in FIG. 1A, a gate electrode 20 is formed on a substrate 10, and a gate insulating film 25 is formed on the entire surface of the substrate including the gate electrode 20.
Next, as shown in FIG. 1B, after an active layer 30a, an etch stopper layer 40a, and a heat transfer layer 45 are sequentially deposited on the gate insulating layer 25, a laser is irradiated to crystallize the active layer 30a. 
The heat transfer layer 45 absorbs energy of the laser and transfers the absorbed energy to the active layer 30a. In more detail, since the active layer 30a does not absorb energy well if the laser is directly irradiated to the active layer 30a, a metal material easy to absorb energy of the laser is used to transfer energy to the active layer 30a. 
The etch stopper layer 40a serves as a stopper during a later etching process, and also serves to prevent reaction between the metal constituting the heat transfer layer and a silicon material constituting the active layer 30a from occurring during laser irradiation.
Next, as shown in FIG. 1C, after the heat transfer layer 45 is removed, the etch stopper layer 40a is patterned to form a predetermined etch stopper 40.
Next, as shown in FIG. 1D, an ohmic contact layer 50a and a source/drain electrode layer 60a are sequentially deposited on the entire surface of the substrate including the etch stopper 40.
Then, as shown in FIG. 1E, the source/drain electrode layer 60a is patterned to form a source electrode 62 and a drain electrode 64, and the active layer 30a and the ohmic contact layer 50a are etched using the source and drain electrodes 62 and 63 as masks to form an ohmic contact layer 50 and an active layer 30 at a predetermined pattern.
Since the etch stopper 40 is not formed at a left region of the source electrode 62 and a right region of the drain electrode 64, the ohmic contact layer 50a and the active layer 30a are etched together. However, since the etch stopper 40 is formed at a channel region between the source electrode 62 and the drain electrode 64, the ohmic contact layer 50a is only etched.
However, the aforementioned thin film transistor according to the related art has the following problems.
If the laser is irradiated during the process of FIG. 1B, serious stress is given to the gate electrode 20, whereby a crack occurs in the gate insulating film 25 formed on the gate electrode 20.
In order to solve the above problem, the gate electrode 20 may be formed at a thin thickness. However, in this case, problems occur in that resistance of the gate electrode 20 increases and on current characteristics of the thin film transistor may be deteriorated.